Non-primate lentiviral vectors offer potentially safer and more convenient gene transfer to human cells than vectors based upon HIV-1. While the primate lentiviruses are derived from lethal human pathogens, there is no evidence that humans can be infected by FIV. Mobilization of HIV vectors in HIV infected individuals, while a possible advantage, raises the concern of transfer of vector to unintended targets, including germ cells. Recently, our studies showed that the only restriction to productive infection of human cells by feline immunodeficiency virus (FIV) is the inactivity of the FIV promoter. The use of a heterologous promoter allows production of FIV-based vectors in human cells (avoiding the risk of contamination by endogenous retroviruses present in non-human cells, while retaining the desirable lenti-retroviral capacity to transduce non-dividing, terminally differentiated cells such as neurons and monocyte-macrophage. However, the relative efficiency of FIV- and HIV-based vector systems for transduction and expression remains to be determined. FIV packaging systems which minimize recombination by use of separate constructs have yet to be developed. The antiviral activity of FIV vectors expressing ribozymes has not been examined, and the mobilization of FIV vectors by HIV or SIV infection has not been studied. The major goals of this project are: [1] To compare transduction efficiency of analogous FIV and HIV based vectors, and the extent to which these vectors are mobilized by HIV or SIV infection in vitro, [2] To develop safe and efficient packaging cell lines for production of high-titer FIV vectors, [3] To compare the efficiency of HIV and HIV expressing anti- HIV or SIV ribozymes to confer antiviral resistance, and [4] To examine the ability of FIV vectors to transduce primate hematopoietic stem cells capable of repopulation in macaque models, in collaboration with Project III. This project should provide valuable information needed to make a rational choice of FIV or HIV vectors for clinical applications such as gene therapy for HIV infection.